1. Field of the Invention
The present invention relates to a peak suppression technique, and to a technique effectively applicable to a peak suppression technique, et cetera, for an input signal to a transmission amplifier for use in a digital wireless telecommunication, et cetera for example.
2. Description of the Related Art
A linear amplification is required to a transmission amplifier for use in the currently practiced major digital wireless telecommunication systems.
Meanwhile, in a multi-carrier telecommunication system such as the Orthogonal Frequency Division Multiple Access (OFDM) system, a known fact is that a high peak power is often generated at the time of synthesizing a transmission signal.
Generally, as a Peak-to-Average Power Ratio (PAPR) becomes large, a back-off power of a transmission amplifier needs to be large so as to operate it in a linear range, resulting in degrading an efficiency of the transmission amplifier.
In order to prevent it, various peak suppression techniques have been proposed for suppressing a peak component relating to a transmission signal input from a digital signal generation unit to a digital-to-analog (D/A) conversion unit at a front stage of a wireless transmission unit.
Conventional known peak suppression techniques include a clipping method limiting the maximum value of a peak wave form to a suppressed threshold value or smaller, and a window function method multiplying a coefficient so that the maximum value of a peak wave form is suppressed to a suppression threshold value or smaller.
FIG. 1 is a graph showing how a time signal is suppressed in the case of applying each of the peak suppression techniques to a transmission signal, with the horizontal axis indicating time and the vertical axis indicating voltage. FIG. 2 is a graph showing how a spectrum appears in each of the cases of the original time signal, clipping method and window function method, with the horizontal axis indicating frequency and the vertical axis indicating power.
In the above described clipping method, only the necessary minimum transmission signal is suppressed so as to shave off a part exceeding a suppression threshold value and therefore the shaved off signal is a small portion, thereby making a small degradation of a reception characteristic. On the other hand, high frequency components are generated because the ends of the suppression are not smooth, thereby degrading a spectrum characteristic (i.e., a distortion characteristic). For preventing this, a common method is a parallel use of a low pass filter (LPF) which, however, is faced with a technical problem of increasing a circuit size for including the LPF and of reproducing the once clipped peak as a result of the filtering.
Meanwhile, a window function method suppresses a peak by multiplying it by a coefficient whose distribution is defined by the window function as shown by FIG. 1. In this case, a degradation of a spectrum characteristic is comparably smaller than the case of only using the clipping method because the ends of a suppression range can be relatively smoother, which, however, requires a multiplication of a window having a long time-width to some extent by an original signal, resulting in cutting that much signal off the original signal and an accordingly great degradation of the reception characteristic.
As far as using each of these methods independently, the relationship of a reception characteristic and a degradation of a spectrum becomes a subject of trading off and therefore what is demanded is a method for adequately adjusting the balance between the aforementioned two aspects suitably to a system to be applied to.
A patent document 1 has disclosed a technique for generating a peak factor corresponding to a ratio of a transmission target signal level threshold value to a peak level of a transmission target signal in order to judge the peak of the transmission target signal and multiplying a result of weighting the peak factor by a predefined window function, as a peak suppression coefficient, by the level of the transmission target signal together, thereby suppressing the peak.
The technique disclosed by the patent document 1, using only the window function for carrying out the suppression, cannot solve the above described technical problem which is specific to the window function.
[Patent document 1] Laid-Open Japanese Patent Application Publication No. 2005-20505